Chemical doping to control the in-situ formed doping structure in light-emitting electrochemical cells

The initial operation of a light-emitting electrochemical cell (LEC) constitutes the in-situ formation of a p–n junction doping structure in the active material by electrochemical doping. It has been firmly established that the spatial position of the emissive p–n junction in the interelectrode gap has a profound influence on the LEC performance because of exciton quenching and microcavity effects. Hence, practical strategies for a control of the position of the p–n junction in LEC devices are highly desired. Here, we introduce a “chemical pre-doping” approach for the rational shifting of the p–n junction for improved performance. Specifically, we demonstrate, by combined experiments and simulations, that the addition of a strong chemical reductant termed “reduced benzyl viologen” to a common active-material ink during LEC fabrication results in a filling of deep electron traps and an associated shifting of the emissive p–n junction from the center of the active material towards the positive anode. We finally demonstrate that this chemical pre-doping approach can improve the emission efficiency and stability of a common LEC device.

www.nature.com/scientificreports/ In the present study, we demonstrate that a rational and controlled shift of the position of the emissive p-n junction in LEC devices can be attained by a new concept, viz. "chemical pre-doping" of the organic semiconductor already during the LEC fabrication. We specifically show that the addition of a soluble strong chemical reductant, in the form of the neutral organic molecule termed reduced benzyl viologen (r-BV 0 ), to the active-material ink results in an electron transfer and an associated filling of deep electron traps of a common emissive organic semiconductor termed Super Yellow. By combined experimentation and simulation, we further show that this chemical pre-doping approach enables for a significant and rational spatial shift of the emissive p-n junction, that the magnitude of this shift can be controlled by the concentration of the chemical dopant, and that this addition can enable for an improved device performance in the form of an increased emission efficiency and stability.
We herein synthesized r-BV 0 by reducing a benzyl viologen dichloride salt (BV 2+ Cl -2 ) with the aid of a large surplus of a NaBH 4 salt as the reducing agent [35][36][37][38][39] in a two-component water:toluene solution, as schematically depicted in Fig. 1a. The exothermic reaction proceeds in two reduction steps, with the initial step featuring an electron transfer from NaBH 4 to the nitrogen on one of the two pyridine rings of the BV 2+ di-cation for the formation of a r-BV + mono-cation; while the final step constitutes an electron transfer from NaBH 4 to the nitrogen  www.nature.com/scientificreports/ on the second of the pyridine rings for the formation of the desired neutral r-BV 0 end product. The chemical structures of these three different oxidation states of the benzyl viologen (BV) molecule, and their associated colors, are displayed in Fig. 1b. The progression of this chemical reaction can be observed visually, since the two reactants (BV 2+ Cl -2 , and NaBH 4 ) are colorless, while the r-BV + intermediate is colored violet, and the final r-BV 0 product is pale yellow 29,35,[38][39][40][41][42][43][44][45] . The conclusion of the reaction can accordingly be determined by the reaction solution shifting color from violet to yellow (see Fig. S1), in combination with the disappearance of the H 2 gas bubbles from the reaction solution (see Fig. 1a). The completion of this double-reduction reduction was further confirmed by the recording of a silent EPR spectrum (data not shown). The single-reduction product r-BV + would in contrast have exhibited a characteristic EPR spectrum 46 . The employment of the poorly miscible two-component water:toluene reaction solution enabled for the practical collection of the neutral r-BV 0 product from the upper (and lighter) toluene phase, since both reactants and all other non-gaseous products are solely soluble in the lower water phase. More details on the synthesis are available in the Methods section.
The extracted "doping solution" (i.e., r-BV 0 dissolved in toluene) exhibited a r-BV 0 solute concentration of ~ 4 g·l −1 . This solution was used as the chemical reductant in the formulation of the "active-material inks", which also comprised the electroluminescent and polymeric organic semiconductor termed Super Yellow, the ion transporter hydroxyl-capped trimethylolpropane ethoxylate (TMPE-OH), and the salt KCF 3 SO 3 in a constant mass ratio of 1:0.1:0.03. The r-BV 0 concentration was varied in the different inks, as quantified by the mass fraction of r-BV 0 with respect to Super Yellow. Figure 1c is an electron-energy diagram, which presents values for the HOMO of r-BV 0 to the left and the LUMO, HOMO and an electron-trap level of Super Yellow 47-49 to the right. The HOMO value for r-BV 0 was derived from a cyclic voltammetry (CV) measurement vs. the standard hydrogen electrode (SHE), and by using the convention that the SHE electrode is offset by 4.44 eV from the vacuum level. 30,50,51 The HOMO and LUMO of Super Yellow were similarly determined from CV measurements, 52 while a major and generic electron trap level in organic semiconductors was identified and rationalized by Blom et al. 47,49 A recent LEC study further suggest that the inclusion of the TMPE-OH ion transporter into the active material of our LEC devices can result in the formation of a significant concentration of additional electron traps 25 , but their exact energy level is currently unknown. Nevertheless, the presented electron-energy diagram implies that an electron transfer from the HOMO of r-BV 0 to the established electron-trap level of Super Yellow should be possible, as indicated by the arrow in Fig. 1c.
The photoluminescence quantum yield (PLQY) is a sensitive indicator of the existence of "trap impurities" in organic semiconductors, since the singlet exciton typically diffuses a significant distance of ~ 10 nm during its lifetime in neat organic semiconductors 53 , and since the exciton commonly is efficiently quenched by such trap impurities 54,55 . Figure 2a presents the evolution of the PLQY of the active-material film as a function of the added r-BV 0 concentration. Interestingly, we find that the PLQY first increases up to ~ 2 mass% of r-BV 0 , and thereafter decreases monotonously with further increasing r-BV 0 concentration. The initial increase of the PLQY implies that the first added r-BV 0 molecules effectively eliminates "dark" electron traps, presumably by donating electrons and thereby filling of electron trap levels, as schematically depicted in Fig. 1c.
The influence on the electronic conductivity of Super Yellow by the addition of r-BV 0 was investigated by the fabrication and characterization of glass/ITO/Al/(Super Yellow + r-BV 0 )/Ca/Al electron-only devices. Figure 2b shows that the current density increases by more than one order of magnitude over the entire probed voltage interval when the r-BV 0 concentration is increased from 0 to 3 mass%; but that trend is reversed at higher r-BV 0 concentrations exceeding 3 mass%, for which the electronic conductivity is reverting back to the lower level of intrinsic Super Yellow. www.nature.com/scientificreports/ Accordingly, the summary conclusion from the data presented in Figs. 1c and S1 is that the first addition of r-BV 0 molecules (up to 2-3 mass%) results in a filling of electron traps of Super Yellow, which is manifested in an increased electron mobility (and thereby an improved electronic conductivity) 56 and an improved PLQY. A higher concentration of added r-BV 0 molecules results in transport-and emission-damaging effects that obviously should be avoided in devices. We now turn our attention to the investigation of the effects of this dopantenabled electron-trap filling approach on the performance of LEC devices. Figure 3a presents the simulated steady-state exciton profile in the interelectrode gap for ITO/active-material/ Al LECs, which are solely distinguished by the non-filled electron-trap concentration on the organic semiconductor in the active material, as identified in the inset. The trap-free electron/hole mobility ratio was set to 3 in the simulation, and the anodic (cathodic) interface is positioned at a normalized interelectrode position of 0 (1). The peak exciton formation rate can be considered to represent the position of the emissive p-n junction.
The simulation shows that the position of the p-n junction shifts from the center of the active material towards the positive anode with decreasing concentration of electron traps, i.e., with increasing concentration of chemical-reductant molecules, as highlighted by the arrow. This behavior is explained by that the steady-state position of the p-n junction in LEC devices is determined by the effective electron/hole mobility ratio, with a higher electron (hole) mobility resulting in a p-n junction positioned closer to the positive anode (negative cathode). The filling of electron traps will increase the effective electron mobility, and it is accordingly expected to result in the observed shift of the p-n junction towards the positive anode.
We have experimentally investigated the effects of the addition of the r-BV 0 chemical reductant on the p-n junction position by first measuring the angle-dependent EL spectrum and intensity for ITO/(Super Yellow + TMPE-OH + KCF 3 SO 3 + r-BV 0 )/Al LECs and by thereafter simulating the same data. The sole free parameter in the simulation was the position of the emissive p-n junction, and by systematically determining the value for www.nature.com/scientificreports/ this parameter that produced the best agreement between the measured and simulated data, we could establish the position of emissive p-n junction with high accuracy 25,57,58 . A number of visual examples of this best-fit procedure for the identification of the p-n junction position are displayed in Fig. S3, while the Methods section provides more details on its practical execution. Figure 3b presents the experimentally determined steady-state position of the p-n junction in the interelectrode gap as a function of the r-BV 0 concentration. The LEC device void of r-BV 0 exhibits a steady-state p-n junction in the exact middle of the active material at 0.50, while the addition of the r-BV 0 chemical reductant to the active material results in a gradual and significant anodic shift of the p-n junction position to 0.43 at [r-BV 0 ] = 1 mass% and 0.39 at [r-BV 0 ] = 3 mass%. This experimental finding is thus in excellent agreement with the simulation results displayed in Fig. 3a. At a higher r-BV 0 concentration ([r-BV 0 ] ≥ 5 mass%) the anodic shift is halted, which is in line with our earlier finding that these higher r-BV 0 concentrations cause transport (and emission) limiting effects.
We have finally investigated the effects of this chemical pre-doping approach, and its induced p-n junction shift, on the performance of LEC devices. Figure 3c,d display the measured voltage and luminance transients, respectively, of representative ITO/(Super Yellow + TMPE-OH + KCF 3 SO 3 + r-BV 0 )/Al LEC devices during electrical driving by a constant current density of 7.75 mA·cm -2 . The r-BV 0 concentration is identified in the inset of Fig. 3c. The corresponding shorter-time luminance and current-efficacy transients are displayed in Fig. S4. We have measured eight independent devices for each r-BV 0 concentration, and Fig. S2 and Table S1 present the average and standard deviation for key device metrics.
All investigated LEC devices feature a decreasing voltage and an increasing luminance during the initial operation, and a fast turn of < 2 s to a luminance exceeding 100 cd·m −2 . This implies that the LEC-characteristic in-situ electrochemical doping capacity of the organic semiconductor Super Yellow is not significantly damaged or affected by the addition of r-BV 059 . We find that the LEC devices with a r-BV 0 concentration of ≤ 3 mass% exhibit a markedly better performance than the LECs with a higher r-BV 0 concentration (see also Fig. S2 and Table S1). This supports our earlier finding that a too high r-BV 0 concentration causes transport and emission limiting effects within the active material.
Importantly, we find that the LEC with 1 mass% r-BV 0 exhibits ~ 10% higher peak luminance and ~ 70% longer operational lifetime than the reference LEC void of r-BV 0 , as gleaned from the statistical analysis presented in Table S1. (The operational lifetime is here defined as the total time that the device emits with a luminance exceeding 100 cd·m −2 ). We assign these improvements to the combined effects of a slightly increased PLQY (Fig. 2a), and the associated suppression of weakly emissive trap-assisted electron and hole recombination 49 , in combination with the anodic shift of the emissive p-n junction following the addition of 1 mass% r-BV 0 to the active material (Fig. 3b). An anodic shift of the exciton population can for this particular device be attractive from an emission-efficiency viewpoint since the ITO anode is a less potent exciton quencher than the Al cathode 60,61 . Accordingly, a shift of the emissive p-n junction from the center of the active material towards the positive ITO anode can be expected to result in lowered losses due to exciton-electrode quenching. We finally note that exciton quenching can result in severe self heating 62,63 and/or the formation of highly localized high-energy species on the organic semiconductor, which in turn can cause material and device degradation. Thus, the suppression of exciton quenching reactions by the electron trap filling of the organic semiconductor by the chemical reductant can also rationalize the observed prolongation of the device lifetime.

Conclusions
We report on chemical pre-doping as a novel and functional tool for the rational adjustment of the doping structure and the p-n junction position in LEC devices. Such a tool is much desired, since it is well established that the in situ formed doping structure has a strong influence on the LEC performance. We specifically synthesize a solution-processible r-BV 0 chemical reductant with a high HOMO level, which enables for effective electron transfer to a generic electron trap level of organic semiconductors. By a combination of simulations and experiments, we show that the addition of this chemical reductant to the active material of a common LEC device results in a shift of the emissive p-n junction from the center of the active material towards the transparent positive anode, and that such an optimized addition can result in a markedly improved device performance.

Methods
Synthesis of the chemical reductant. The overall reaction scheme for the chemical synthesis of the reduced benzyl viologen (r-BV 0 ) chemical reductant is displayed in Fig. 1a. The reaction is initiated by mixing 20 mg (0.05 mmol) benzyl viologen dichloride salt (BV 2+ Cl − 2 , C 24 H 22 N 2 2+ Cl − 2 , M W = 409.35 g·mol −1 , 97%, Merck, GER) with 4 g (0.1 mol) NaBH 4 salt (M W = 37.83 g·mol −1 , powder, ≥ 98.0%, Merck, GER) in 10 ml (0.6 mol) deionized water, followed by the addition of 5 ml (0.05 mol) of toluene. The B 2 H 6 product is quickly reacting with water to form B(OH) 3 boric acid and H 2 gas, with the latter being observed as gas bubbles escaping the reaction liquid.
At the end of the reaction, the water and toluene solvents are visibly separated, with the lower-density toluene floating on top of the water. Both reactants (i.e. the two salts BV 2+ Cl -2 and NaBH 4 ) and all non-gaseous products are highly soluble in water but effectively insoluble in toluene 28 , with the exception being the neutral r-BV 0 reaction product that instead is highly soluble in toluene 29,35,[38][39][40][41] . This distinct dissolution capacity enables for the facile collection of r-BV 0 at the end of the reaction through the extraction of the lighter toluene phase with a syringe. Figures 1b and S1 reveal that the progression of the reaction can be visualized through the change in color that accompanies the two-step reduction of the BV molecule. The original BV 2+ di-cation reactant is colorless, while the intermediate r-BV + mono-cation is violet, and the final neutral r-BV 0 product is pale yellow in color. We could thus determine the conclusion of the reaction by the time at which the toluene phase had changed color to Scientific Reports | (2023) 13:11457 | https://doi.org/10.1038/s41598-023-38006-y www.nature.com/scientificreports/ yellow and when the formation of H 2 gas bubbles had ceased. The time for complete reaction was approximately three days. It is notable that the reaction was executed under ambient air at room temperature without the use of a catalyst. The r-BV 0 concentration in the extracted 5 ml toluene solution is ~ 4 g·l −1 , and from here on we refer to it as the "doping solution".  For the LEC devices, pre-patterned indium-tin oxide (ITO) coated glass substrates (ITO thickness = 145 nm, ITO sheet resistance = 20 Ω/sq, substrate area = 30 × 30 mm 2 , Kintech, CHN) were cleaned by sequential 15 min ultrasonic treatment in detergent (Extran MA 01, Merck, GER), deionized water (two cycles to completely remove detergent), acetone, and isopropanol, and then dried in an oven at 120 °C for > 30 min. The ITO substrates were treated by a UV/ozone treatment for 10 min prior to spin coating to improve the ink wettability.

Ink formulation.
For the angle-dependent LEC measurement, the active-material ink based on the higher viscosity 10 g·l −1 Super Yellow master solution was spin coated with a spin speed of 2000-2300 rpm for 120 s, for the attainment of a dry active-material film thickness of ~ 175 nm. For the forward-luminance LEC measurement, the activematerial ink based on the lower-viscosity 8 g·l −1 Super Yellow master solution was spin coated at a spin speed of 1500-2000 rpm for 60 s, for the attainment of a dry active-material film thickness of ~ 110 nm. The spin-coated active-material films were dried at 70 °C for 1 h on a hot plate.
The 100-nm thick Al top electrodes were deposited by thermal evaporation in a vacuum chamber (p ≤ 5 × 10 -4 Pa), with a quartz crystal monitoring the Al evaporation rate (5-7 Å·s -1 ) and the Al thickness. A shadow mask positioned in between the evaporator source and the LEC device defined the size and shape of the Al electrodes. The spatial overlap between the Al and ITO electrodes defined four 2 × 2 mm 2 LEC devices on each substrate. The Al top electrode was invariably biased as the negative cathode in all measurements.
The electron-only glass/ITO/Al/(Super Yellow:r-BV 0 )/Ca/Al devices were fabricated in a similar manner, but with a 100 nm thick Al electrode thermally evaporated onto the ITO surface, a 100 nm thick Super Yellow:r-BV 0 blend film spin-coated onto the Al, and a 20 nm thick Ca layer thermally evaporated on the blend film.
The forward-luminance and the electron-only measurements were performed on non-encapsulated devices in the glove box. The forward luminance and the voltage were measured at a constant current density of 7.75 mA·cm −2 , using a computer-controlled OLED lifetime setup (M6000 PMX, McScience, KOR).
The angle-dependent measurements were conducted under ambient air, and these devices were therefore encapsulated by attaching a 24 × 24 mm 2 glass cover slide (Menzel GmbH, GER) onto the Al cathode side with UV-curable epoxy (E132-60 mL, Ossila). The epoxy was cured by exposure to UV light (λ peak = 365 nm, power density = 80 mW mm −2 , UV-Exposure Box 1, Gie-Tec) for 15 min. The angle-dependent EL measurements were performed with a custom-built spectrogoniometer setup, essentially comprising a rotation stage, a stepper motor, and a fiber-optic CCD-array spectrometer (Flame-S, Ocean Optics) 22 . The forward emission was measured at 0°, and the viewing angle was varied from −80° to 80° in 10° steps. The spectrogoniometer was controlled with a Python-based virtual instrument using a Raspberry Pi 400. For these measurements, the devices were driven by a constant current density of 25 mA·cm -2 at a 21 V compliance voltage, with the current supplied and the voltage measured with a source measure unit (Keithley 2400).
The optical simulations were carried out with a commercial software (Setfos 5.2, Fluxim AG, CHE). The position of the emissive p-n junction within the active material was determined by minimizing the root mean square error between the simulated and the measured angle-dependent EL data. Further details on this procedure can be found in Ref 22 . The exciton formation rate profile in the interelectrode gap was determined with the drift-diffusion module of the same software (Setfos 5.3, Fluxim AG, CHE), and the simulated three-layer device featured an ITO anode (thickness = 145 nm), an active material (thickness = 150 nm), and an Al cathode. The parameter values for this device structure were gleaned from literature references 25,64 and are listed in Table S1 in the Supporting Information. The simulated devices were driven by a constant voltage of 3 V.

Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.